C. B. Davenport 19 



mentally different possible theory of adaptation, and this is that the structure existed 

 first and a fitting environment was sought or fallen into by the species having the 

 peculiar bodily condition. Thus the adaptive result is, on this theory, not due to a 

 selection of structure fitting a given environment, but, on the contrary, a selection of 

 an environment fitting a given structure. I shall now consider some special cases 

 that are best explained on this theory. Thus, Eigenmann (1899) shows that the cave 

 fishes, which in many points show an adaptation to the cave environment, are not to 

 be thought of as having accidentally got into caves and as having subsequently gained 

 a structure fitting them for that environment. But, on the contrary, as they all belong 

 to one family, their getting into the caves was evidently not an accident. Moreover, 

 this family includes species that are structurally especially fitted for cave life, even 

 when they occur in regions where there are no caves and never have been any. They 

 shun the light, and live in crevices and under stones. Their bodily conditions fit 

 them for cave life and when, in their constant search for dark holes, some of them 

 succeeded in getting into caves, they naturally thrived there. 



Again, in many cases of parasitism among snails the radula is known to be absent 

 altogether, and this has been accounted for by Cooke (1895) on the hypothesis that 

 these snails lost their teeth through disuse. However, it is pointed out as a curious 

 fact that the same absence of a radula occurs in species of Eulima known to be not 

 parasitic. Cooke suggests the hypothesis that in cases like this the form must have 

 derived from parasitic ancestors. It is equally probable that Eulima is a mollusc 

 that will probably soon be driven to parasitism because it has no radula. 



Now, that which is true for the cave animals, and probably true for edentulate 

 snails, is illustrated time and time again in the animals of the beaches. We have seen 

 that the Anurida are covered with fine hairs, which enable them to float upon the tide 

 and thus keep them from drowning. Are not the fine hairs a remarkable adaptation 

 to the necessities of the situation ? They certainly are, but the probability is that the 

 hairs were not developed to meet this situation at all ; at least, such a coating of fine 

 hairs is widespread among Collembola, and the hairs subserve a variety of functions. 

 Thus, Schaffer (1898) finds that the long hairs protect the animal against the action 

 of the sun's rays in the case of certain species that live on leaves; and the importance 

 of such protection would seem to be great, for Absolon (1900) finds that certain cave 

 forms of Collembola, which, so far as he describes them, seem to be scantily covered 

 with hairs, are killed by a few minutes of exposure to sunlight. Hairs are, we may 

 then say, common occurrences on the thin-skinned Collembola. The hairs are impor- 

 tant to keep the thin skin from desiccation. Because the skin is thin, the Collembola 

 favor damp or wet places ; just because they are covered with hairs, they can float on 

 the water; just because they can float on the water, they can live on the lower beach. 

 Also, they find here their appropriate food. Having by some means got to the beach, 

 they remain there, because they find the conditions existing on the beach peculiarly 

 suitable. 



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